Wireless communication networks are widely deployed to provide various communication services such as voice, packet data, multi-media broadcast, and text messaging. These wireless communication networks may be multiple-access systems capable of supporting communication for multiple users by sharing the available network resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems. A CDMA system may implement Wideband CDMA (W-CDMA) or cdma2000. W-CDMA is described in documents from 3rd Generation Partnership Project (3GPP). CDMA2000 is described in documents from a consortium named “3rd Generation Partnership Project 2” (3GPP2). 3GPP and 3GPP2 documents are publicly available. WiMAX (Worldwide Interoperability for Microwave Access)-based systems are being designed and developed for operation in licensed bands, such as 2.3 GHz, 2.5 GHz, 3.3 GHz, 3.5 GHz etc.
A remote unit can move across a wireless communication network accessing different network nodes. To access each network node, the remote unit has to communicate with the respective network node using a ranging procedure. Existing ranging procedures include Fast Ranging or Handoff Ranging. When a handoff of a remote unit is desired, the remote unit may send a request message to a current serving network node. The request message notifies the current serving network node to initiate a handoff procedure. The serving network node, in response to the request message, may notify a target network node about the remote unit's attempt to handoff to the target network node. This notification to the target network node, via the backbone network, prepares the target network node for the handoff of the remote unit. The target network node may then allow non-contention-based Initial Ranging opportunities and a dedicated bandwidth allocation for the remote unit to support handoff ranging.
However, handoff ranging has high latency due to the fact that the remote unit has to initiate bandwidth request to the serving network node and then proceed in a similar manner to initial network entry. In addition, handoff ranging does not provide a unique footprint for the remote unit. This opens up more vulnerability in an extreme case where ranging codes are compromised by a hacker who intends a denial of service on the ranging region.
Fast ranging on the other hand increases resource utilization due to pre-allocated time frames for the remote unit. Most remote units in handoffs may not complete the handoff within the allocated time frames. For example, FIG. 1 illustrates a basic operation of a co-ordinated handoff. Under the basic operation of FIG. 1, a remote unit works through one of 3 Fast Ranging Opportunities that have a handoff latency of 45 ms, 60 ms, or 75 ms depending on the Fast Ranging Information Element (IE) that the remote unit used for the handoff. If any of these opportunities are missed then the remote unit has to perform Handoff Ranging contending for one of many shared Handoff Ranging Codes. FIG. 1 illustrates both the Fast Ranging as well as the Handoff Ranging latency. The following is a detailed description of the signaling flow of FIG. 1, with reference to the individual signaling instances labeled in FIG. 1:
Frame 2 (at remote unit): The remote unit transmits a handoff request (MSHO REQ) to a serving network node.
Frame 3 (at serving network node): The serving network node, in response to the MSHO REQ, transmits a prenotification request (Prenot REQ) to a target network node. The Prenot REQ notifies the target network node about remote unit's attempt to conduct the handoff with the target network node.
Frame 15 (at target network node): The target network node, in response to the Prenot—REQ, transmits a Prenot response (Prenot RSP) to the serving network node.
Frame 15 (at serving network node): The serving network node, in response to the Prenot-RSP, transmits a Handoff advise (HO ADVISE) to the target network node. The HO ADVISE indicates the target network node that remote unit has left for the target network node.
Frame 16 (at target network node): The target network node initiates a Fast Ranging Timer at the beginning of Frame 16. The Fast Ranging Timer takes into account the overheads involved in handover messaging at the serving network node. The Fast Ranging Timer continues until the remote unit leaves the coverage zone of the current serving network node and enters the coverage zone of the target network node.
Frame 16 (at serving network node): The serving network node transmits a handoff response (BSHO RSP) to the remote unit, which is received on frame 17 at the remote unit.
Frame 18 (at remote unit): The remote unit receives an allocation for handoff indication (Alloc for HO IND)
Frame 18 (at remote unit): The remote unit, in response to the allocation for the HO IND, transmits a handoff indication (HO IND) to the serving network node.
Frame 19 (at serving network node): The serving network node, after receiving the HO IND from the remote unit, transmits a handoff confirm (HO Confirm) to the target network node, which is received on frame 20 at the target network node.
Frame 23 (at target network node): The target network node stops the Fast Ranging Timer at the beginning of the Frame 23. The target network node then transmits an allocation for a first fast ranging to the remote unit. At this moment, the remote unit has left the coverage area of the serving network node and enters the coverage area of the target network node.
Frame 24 (at remote unit): The remote unit may either respond to the first fast ranging allocation by sending a ranging request (RNG REQ), as shown with dotted line, or may miss the opportunity to respond to the first fast ranging allocation. If the remote unit responds to the fast ranging allocations, the RNG REQ is then received by the target network node at Frame 25.
The remote unit may miss the opportunity to respond to a fast ranging allocation when the remote unit does not reach the target network node in the time specified for handoff by the target network node during the exchange of messages with the serving network node. The reasons for the remote unit to not reply to the fast ranging allocations may include cell topology, slow handoffs where the remote unit is unable to synchronize with the target network node due to obstructions, or change in RF conditions. Examples for such kind of obstructions may include buildings, slow moving traffic, or weather conditions during the handoff. In this case, the timing between the target network node and the remote unit is not synchronized and the target network node may waste the network resources to send fast ranging allocations to the remote unit.
Frame 26 (at target network node): After receiving the RNG REQ, the target network node transmits a ranging response (RNG RSP) to the remote unit.
If the remote unit is not able to utilize the opportunity of the first fast ranging allocation, the target network node may then transmit more fast ranging allocations to the remote unit. The number of fast ranging allocations sent to the remote unit may depend on the available network resources with the target network node. As shown in FIG. 1 the target network node transmits three fast ranging allocations to the remote unit in order for the remote unit to start fast ranging with the target network node and complete the handoff. The handoff latency from the transmission of the first fast ranging allocation to the third fast ranging allocation may increase from 45 ms to 75 ms.
Frame 32 (at remote unit): If the remote unit misses the third fast ranging allocation also, the remote unit then transmits a handoff ranging (HO RNG) code to the target network node, which is received by the target network node at Frame 33. In this case, the remote unit now has to perform Handoff Ranging to complete the handoff with the target network node. The remote unit selects the handoff ranging code from a plurality of handoff ranging codes that are broadcasted continuously by the serving network node. The selection of the handoff ranging code is based on the selection of a target network node as a potential candidate for handoff by the remote unit. This selection may be performed by the internal circuitry of the remote unit or the remote unit may obtain the information regarding the best candidate for handoff from the serving network node. The remote unit may then utilize the handoff ranging code of that target network node. The handoff ranging codes are broadcasted by the serving network node to all the remote units that communicating with the serving network node. Each handoff ranging code identifies one target network node as a potential candidate for handoff. These codes are Code Division Multiple Access (CDMA) codes that are assigned by every network node to facilitate handoffs.
Frame 34 (at target network node): After the target network node receives the handoff ranging code, the target network node provides timing and power adjustments that are required between the target network node and the remote unit. The target network node transmits the timing and power adjustments as a ranging response (RNG RSP) continue message to the remote unit.
Frame 35 (at remote unit): Following the corrections received as timing and power adjustments, the remote unit transmits the HO RNG code again to the target network node.
Frame 37 (at target network node): The target network node, after receiving the HO RNG code, transmits a RNG RSP success with an allocation for the remote unit to send a RNG REQ.
Frame 39 (at remote unit): The remote unit transmits a RNG REQ to the target network node, which is received by the target network node at Frame 40. The RNG REQ includes a Handoff ID and serving network node information that identifies the remote unit at the target network node.
Frame 41 (at target network node): The target network node transmits the RNG RSP to the remote unit, completing the handoff procedure. However, by this point in time the handoff latency has increase to 120 ms. Due to this high latency, in some cases, a subscriber may experience additional delay if timing and power adjustments are not made. Moreover, the subscriber has to contend for Ranging codes in order to complete the handoff.
Therefore, as alluded to above, Fast Ranging is not an optimized solution to complete the handoff and in most cases it wastes Over The Air Radio resources. Fast ranging provides unsolicited allocations for the remote unit. Since these unsolicited allocations or pre-set allocations utilize a substantial amount of bandwidth, the target network node stops these allocations after a predetermined number of allocations. Total Fast Ranging duration includes the Fast Ranging Timer during which the target network node waits for the remote unit to arrive under the target network node's coverage zone and the pre-set allocations for the remote unit.
Fast ranging may provide a latency of 45 ms, in contrast to a latency of 120 ms in the handoff ranging, however Fast Ranging does not provide Timing and Power Corrections, something that is crucial in keeping a call active with changing RF conditions that typically happen in handoff scenarios. Handoff Ranging on the other hand has a higher latency, such as 120 ms in contrast to a latency of 45 ms in Fast Ranging, but covers for the Timing and Power Corrections which is better in sustaining the link during the handover. If the remote unit does not initiate fast ranging within the allocated time frame, the remote unit then has to perform Handoff Ranging.
Accordingly, there is a need for an improved and more efficient method and apparatus for optimizing network entry during handoffs in a wireless communication network.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.